﻿34 
  Mr. 
  C. 
  F. 
  Brush 
  on 
  the 
  Transmission 
  of 
  

  

  In 
  this 
  conclusion 
  Sir 
  Wm. 
  Crookes 
  was, 
  I 
  think, 
  wrong. 
  

   I 
  find 
  that 
  the 
  curve 
  representing 
  the 
  rate 
  of 
  cooling 
  does 
  

   not 
  break 
  down 
  materially' 
  at 
  pressures 
  as 
  low 
  as 
  a 
  twentieth 
  

   of 
  a 
  millionth. 
  

  

  My 
  own 
  investigations 
  on 
  " 
  The 
  Transmission 
  of 
  Radiant 
  

   Heat 
  by 
  Gases 
  at 
  Varying 
  Pressures 
  " 
  form 
  a 
  part 
  of 
  a 
  

   general 
  study 
  of 
  the 
  properties 
  of 
  high 
  vacua, 
  in 
  which 
  I 
  

   have 
  long 
  been 
  engaged, 
  and 
  which 
  is 
  yet 
  far 
  from 
  being 
  

   completed. 
  

  

  In 
  the 
  course 
  of 
  my 
  w 
  r 
  ork 
  it 
  became 
  necessary 
  to 
  know 
  

   how 
  much 
  of 
  the 
  heat 
  communicated 
  by 
  a 
  good 
  radiating 
  

   body 
  at 
  ordinary 
  temperatures, 
  to 
  a 
  neighbouring 
  body 
  at 
  a 
  

   slightly 
  lower 
  temperature, 
  through 
  an 
  intervening 
  gas, 
  is 
  

   transmitted 
  by 
  the 
  so-called 
  aether, 
  and 
  how 
  much 
  by 
  the 
  gas 
  ; 
  

   and 
  whether 
  any 
  of 
  that 
  transmitted 
  by 
  the 
  gas 
  is 
  com- 
  

   municated 
  otherwise 
  than 
  by 
  the 
  process 
  of 
  convection. 
  Also 
  

   why, 
  and 
  to 
  what 
  extent, 
  do 
  the 
  gases 
  differ 
  from 
  each 
  other 
  

   in 
  their 
  heat 
  transmitting 
  capacities. 
  

  

  In 
  the 
  drawings 
  herewith, 
  Plate 
  I. 
  is 
  a 
  diagram 
  of 
  the 
  

   apparatus 
  used 
  in 
  my 
  experiments. 
  A 
  is 
  the 
  thermometer 
  

   wdnose 
  cooling 
  was 
  observed. 
  It 
  has 
  a 
  very 
  open 
  scale 
  

   divided 
  into 
  two-tenths 
  degrees 
  C. 
  The 
  zero-point 
  is 
  placed 
  

   a 
  long 
  distance 
  (about 
  170 
  millimetres) 
  above 
  the 
  bulb, 
  for 
  

   obvious 
  reasons. 
  The 
  bulb 
  is 
  cylindrical, 
  about 
  20 
  millim. 
  

   long; 
  and 
  about 
  7 
  millim. 
  in 
  diameter, 
  and 
  is 
  coated 
  with 
  

   lampblack 
  applied 
  with 
  a 
  very 
  thin 
  alcoholic 
  solution 
  of 
  shellac. 
  

   After 
  several 
  hours'' 
  baking 
  at 
  100 
  degrees 
  in 
  a 
  good 
  vacuum, 
  

   this 
  bulb 
  gave 
  constant 
  radiation 
  results. 
  The 
  thermometer 
  

   is 
  suspended 
  by 
  a 
  platinum 
  wire, 
  with 
  its 
  bulb 
  in 
  the 
  centre 
  

   of 
  the 
  large 
  pear-shaped 
  glass 
  bulb 
  B, 
  about 
  112 
  millim. 
  

   in 
  diameter. 
  The 
  stem 
  of 
  the 
  thermometer 
  hangs 
  freely 
  

   in 
  the 
  long 
  neck 
  of 
  the 
  large 
  bulb. 
  I 
  shall 
  hereafter 
  call 
  

   the 
  glass 
  bulb 
  B, 
  the 
  " 
  large 
  radiation 
  bulb/' 
  or 
  simply 
  the 
  

   " 
  large 
  bulb," 
  to 
  distinguish 
  it 
  from 
  a 
  smaller 
  one 
  used 
  later. 
  

   The 
  bulb 
  B 
  is 
  surrounded 
  by 
  a 
  copper 
  tank 
  C, 
  lagged 
  with 
  

   woollen 
  cloth, 
  and 
  filled 
  with 
  crushed 
  ice 
  and 
  distilled 
  water. 
  

   A 
  wire 
  netting 
  C 
  serves 
  to 
  keep 
  some 
  of 
  the 
  ice 
  always 
  

   below 
  the 
  lowest 
  point 
  of 
  B. 
  The 
  tank 
  C 
  is 
  movable 
  on 
  

   vertical 
  guides, 
  whereby 
  it 
  may 
  quickly 
  be 
  raised 
  to, 
  or 
  

   lowered 
  from, 
  the 
  position 
  shown, 
  thus 
  exposing 
  the 
  bulb 
  B 
  

   alternately 
  to 
  the 
  ice-bath 
  and 
  the 
  atmosphere 
  of 
  the 
  labo- 
  

   ratory. 
  The 
  bulb 
  B 
  communicates 
  freely 
  with 
  the 
  large 
  

   barometer- 
  tube 
  D, 
  which 
  is 
  used 
  for 
  measuring 
  all 
  but 
  very 
  

   small 
  pressures. 
  E 
  is 
  a 
  standard 
  boiled 
  barometer, 
  dipping 
  

   into 
  the 
  mercury-cistern 
  F, 
  common 
  to 
  both 
  barometers. 
  

   G 
  is 
  a 
  McLeod 
  gauge 
  giving 
  very 
  accurate 
  measurements 
  of 
  

  

  